Even good intentions may be rubbing your colleagues the wrong way.

While working in the IT department at a large telecommunications firm a few years back, Hayley Chalmers, 38, was tormented by a coworker who regularly left his cell phone at his desk while he was away – with the ringer volume jacked way up high. Three or four times a day, Chalmers and some four dozen annoyed colleagues were treated to a loud, digital rendition of Nikolai Rimsky-Korsakov's "Flight of the Bumblebee." After a few months Chalmers finally confronted her tormentor – to no avail. "He never apologized and just kept doing it," she recalls. "It was maddening."

Nearly 80% of American workers say they are stressed out by at least one aspect of their jobs – with about one in ten saying coworkers cause the most tension, according to a study conducted by Harris Interactive on behalf of Everest College. Those colleagues who talk too much, share too much personal information, gossip often or blame others for their failures are among the most stressful, says career coach Marc Dorio, who conducted an informal survey of employees during a management seminar. Despite all the agony they inflict, stress-inducing workers are rarely asked to change by their exasperated office mates – and that may be an expensive mistake: Stressed-out workers spend nearly 50% more on health care each year than their more-relaxed peers, according to a study published in the Journal of Occupational and Environmental Medicine.

2. "It's your fault I can't squeeze into my skinny jeans anymore."

Cupcakes for your coworker's birthday, cookies to celebrate an upcoming vacation, a crumb cake "just because" -- all these thoughtful gestures may in fact be fueling resentment among some of your coworkers, says career coach David Couper, especially those trying to slim down. Almost half of workers say they have gained weight at their current jobs (up slightly from 2009), with nearly one in five citing "workplace celebrations" like birthdays or potlucks as major contributors to this weight gain, according to 2010 study by CareerBuilder.com. On top of that, 38% of workers say they eat more unhealthy snacks at the office than at home.

All that extra snacking, of course, can lead to serious health problems and higher medical costs. The average overweight person pays $346 more per year on medical-related costs than their healthier peers, according to a study from George Washington University. The average severely obese person pays an additional $1,566 each year.

3. "Stop being such a bully."

More than one in three people has been a victim of bullying in the workplace – from verbal abuse and intimidation to sabotage -- according to a study conducted in 2010 by the Workplace Bullying Institute, a non-profit. In most cases, the bully was a higher-ranking worker, but 28% of the time, it was someone at an equal or lower level. And often such harassment is inadvertent. For example, you may consider the teasing of a colleague good-natured, while it's actually making him or her uncomfortable – not that you'd necessarily know it, says career coach David Couper. Just like at the playground, "people are scared of the consequences of calling a bully out."

If it gets bad enough, it may cost a victim his job. More than 40% of women and 36% of men who had been the victim of bullying said they left their jobs because of it, a 2010 study by the Workplace Bullying Institute found. And that may be the best possible outcome. Workplace bullying can lead to "excessive and prolonged stress," which contributes to a host of health problems, including both physical health issues like heart disease and irritable bowel syndrome, as well as mental health issues like depression and anxiety, says Dr. Joseph Cilona, a Manhattan psychologist. Treatment for depression, for example, can cost as much as $1,000 or more per month for counseling and medication, he says, and many insurance plans limit the number of therapy sessions per year.

4. "I'm sleeping with the boss."

Despite the warnings about office romances, experts say such affairs are still fairly common. Two out of every five employees says they've had at least one office romance; more than one-third of those hooked up with a superior in the company, according to a survey of more than 8,000 workers by CareerBuilder.com. Separately, one-third of executive women said they know a colleague who's had an affair with a boss, according to a 2010 study by the Center for Work-Life Policy.

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